In recent years, the photochemical vapor reaction method (referred to below as the photo CVD method of manufacturing thin films for polycrystalline silicon semiconductor or passivation use and other thin films has attracted considerable attention.
Prior art equipment used in manufacturing this photo CVD method use an arrangement in which a substrate on which the thin film is to be formed is placed inside a reaction chamber having a window that is transparent to ultraviolet radiation. A gas used for forming the thin film is introduced under reduced pressure. Ultraviolet radiation passes through the window transparent to it and causes a photo CVD reaction inside the reaction vessel. The reaction product deposits on the substrate, where it forms a thin film. The window that is transparent to ultraviolet radiation is made of a material that passes ultraviolet radiation easily, such as quartz glass, lithium fluoride or magnesium fluoride.
Methods of increasing the rate of thin film formation including adding mercury into the reaction vessel, together with the gas that is used to form the thin film. To make it possible to form a thin film of large area, the pressure inside the ultraviolet radiation source chamber is decreased, and the window that is transparent to ultraviolet radiation is enlarged.
However, since this photo CVD method requires ultraviolet radiation to be introduced into the reaction vessel through a window that is transparent to ultraviolet radiation, a thin film forms not only on the substrate but also on the window transparent to ultraviolet radiation, so that although sufficient ultraviolet radiation passes through the window into the reaction vessel in the initial stage of the reaction, as the reaction proceeds, a thin film forms on the window, reducing the amount of ultraviolet radiation that passes through; eventually the thin film stops forming on the substrate.
For this reason, when this method is used to form a thin film, there is a limit to thin film thickness; if a film thicker than that limit is to be formed, the window transparent to ultraviolet radiation must be cleaned, and a similar reaction carried out all over again.
This limiting film thickness differs somewhat depending on the type of film being formed; for example it is 800 angstrom units to 1,000 angstrom units for a silicon nitride film, and on the order of 600 angstrom units to 800 angstrom units for an amorphous silicon semiconductor film.
One method that has been proposed for solving this problem is that the inside (reaction vessel side) of the window transparent to ultraviolet radiation be coated, but this has the problem that the oil constituents are drawn into the film being formed by the photo CVD reaction, causing deterioration of the film properties.